How does an Interworx admin handle being the 'external' mail provider?

I have used external mail services before but not the other way around with Interworx. i.e., An existing Siteworx account is relocating to different web and database hosting but they want to keep using the iWorx mail server. Do I need to keep the Siteworx account and its SOA record along with the necessary MX and A records? Which ones to nix are apparent but I’m also having trouble understanding the aliases and/or redirects I need for accessing ‘/webmail’ URL on the iWorx server. Help is appreciated.

yes, i need to know this as well. email being sent shows instead of originating from the domain itself. seems wierd.